Staphylococcus aureus (“S. aureus”) is a bacterium that commensally colonizes more than 25% of the human population. Importantly, this organism is capable of breaching its initial site of colonization, resulting in bacterial dissemination and disease. S. aureus is the leading cause of nosocomial infections, is the most common etiological agent of infectious endocarditis as well as skin and soft tissue infections, and is one of the four leading causes of food-borne illness. Altogether, S. aureus infects more than 1.2 million patients per year in U.S. hospitals. The threat of S. aureus to human health is further highlighted by the emergence of antibiotic-resistant strains (i.e., methicillin-resistant S. aureus (MRSA) strains), including strains that are resistant to vancomycin, an antibiotic considered the last line of defense against S. aureus infection. These facts highlight the importance of developing novel therapeutics against this important pathogen.
S. aureus produces a diverse array of virulence factors and toxins that enable this bacterium to neutralize and withstand attack by different kinds of immune cells, specifically subpopulations of white blood cells that make up the body's primary defense system. The production of these virulence factors and toxins allow S. aureus to maintain an infectious state (see Nizet, “Understanding How Leading Bacterial Pathogens Subvert Innate Immunity to Reveal Novel Therapeutic Targets,” J. Allergy Clin. Immunol. 120(1):13-22 (2007)). Among these virulence factors, S. aureus produces several bi-component leukotoxins, which damage membranes of host defense cells and erythrocytes by the synergistic action of two non-associated proteins or subunits (see Menestrina et al., “Mode of Action of Beta-Barrel Pore-Forming Toxins of the Staphylococcal Alpha-Hemolysin Family,” Toxicol. 39(11):1661-1672 (2001)). Among these bi-component leukotoxins, gamma-hemolysin (HlgAB and HlgCB) and the Pantone-Valentine Leukocidin (PVL) are the best characterized.
The toxicity of the leukocidins towards mammalian cells involves the action of two components or subunits. The first subunit is named class S-subunit (i.e., “slow-eluted”), and the second subunit is named class F-subunit (i.e., “fast-eluted”). The S- and F-subunits act synergistically to form pores on white blood cells including monocytes, macrophages, dendritic cells, and neutrophils (collectively known as phagocytes) (see Menestrina et al., “Mode of Action of Beta-Barrel Pore-Forming Toxins of the Staphylococcal Alpha-Hemolysin Family,” Toxicol. 39(11):1661-1672 (2001)). The mechanism by which the bi-component toxins form pores in target cell membranes is not entirely understood. The proposed mechanism of action of these toxins involves binding of the S-subunit to the target cell membrane, most likely through a receptor, followed by binding of the F-subunit to the S-subunit, thereby forming an oligomer which in turn forms a pre-pore that inserts into the target cell membrane (Jayasinghe et al., “The Leukocidin Pore: Evidence for an Octamer With Four LukF Subunits and Four LukS Subunits Alternating Around a Central Axis,” Protein. Sci. 14(10):2550-2561 (2005)). The pores formed by the bi-component leukotoxins are typically cation-selective. Pore formation causes cell death via lysis, which in the cases of the target white blood cells, has been reported to result from an osmotic imbalance due to the influx of cations (Miles et al., “The Staphylococcal Leukocidin Bicomponent Toxin Forms Large Ionic Channels,” Biochemistry 40(29):8514-8522 (2001)).
In addition to PVL (also known as leukocidin S/F-PV or LukSF-PV) and gamma-hemolysin (HlgAB and HlgCB), the repertoire of bi-component leukotoxins produced by S. aureus is known to include leukocidin E/D (“LukE/D”), leukocidin A/B (“LukAB”) and leukocidin M/F (“LukMF”). Thus, the S-class subunits of these bi-component leukocidins include HlgA, HlgC, LukE, LukS-PV, LukA, and LukM, and the F-class subunits include HlgB, LukD, LukF-PV, LukB, and LukF′-PV. The S. aureus S- and F-subunits are not leukocidin-specific. That is, they are interchangeable such that other bi-component combinations could make a functional pore in a white blood cell, greatly increasing the repertoire of leukotoxins (Meyer et al., “Analysis of the Specificity of Panton-Valentine Leucocidin and Gamma-Hemolysin F Component Binding,” Infect. Immun. 77(1):266-273 (2009)).
Designing effective therapy to treat MRSA infection has been especially challenging. In addition to the resistance to methicillin and related antibiotics, MRSA has also been found to have significant levels of resistance to macrolides (e.g., erythromycin), beta-lactamase inhibitor combinations (e.g., Unasyn, Augmentin), and fluoroquinolones (e.g. ciprofloxacin), as well as to clindamycin, trimethoprim/sulfamethoxisol (Bactrim), and rifampin. In the case of serious S. aureus infection, clinicians have resorted to intravenous vancomycin. However, there have been reports of S. aureus resistance to vancomycin. Thus, there is a need to develop new treatments that effectively combat S. aureus infection.
The present invention is directed to overcoming these and other limitations in the art.